Fiber-reinforced resin matrix composites are widely accepted for use as high strength low weight engineering materials to replace metals in aircraft structural applications and the like. These composite materials may be made by laminating material prepregs comprising high strength reinforcing fibers, such as glass, graphite (carbon), boron, aramid or the like impregnated with a thermoset or thermoplastic resin matrix. Important properties of such material composites are high strength and stiffness and reduced weight.
Prepreg material compositions comprising a blend of polymer resins, and reinforcing fibers are characterized by individual physical and chemical properties of the constituent polymer resins and reinforcing fibers, whereby compositions may be selected for a specific use. Typically, a thermoset resin matrix component is present which confers high solvent resistance, thermal cycling resistance, etc. In addition, a thermoplastic resin component may be added to the thermoset resin to confer higher levels of toughness etc, and reinforcing fibers are present which confer high levels of stiffness and strength.
Composites are traditionally made using material prepregs made up of reinforcing fiber or structural fabrics impregnated with a curable resin matrix composition. Sheets of prepreg materials may be cut to size for laying up, molding, curing and laminating in the construction of a given composite material. Prepreg properties and resulting composite material quality can be controlled to manipulate resulting composite material properties such as toughness, strength and flexibility.
Additives may be added to a base thermoset resin matrix formulation to increase toughness. Suitable additives include thermoplastics, flow modifiers, fillers etc.
Additives may also be added to thermoplastic resins for prepreg impregnation. These additives include plasticizers, fillers, flow modifiers etc.
Different design allowables are used when designing with composite materials depending on the status of stress, the geometry and the boundary conditions that characterize the composite material considered. One such design allowable is notched properties. Notched properties are very important when the designed structure contains holes and when fasteners are used. Notched properties measure of the ability of a given composite material to carry load once a hole is drilled on the load bearing region of the composite material itself. Such notched properties are referred to as Filled Hole Tension and Filled Hole Compression (FHT, FHC), and Open Hole Tension and Open Hole Compression (OHT, OHC). These notched properties are typically the critical design allowables for parts whose thickness is greater than 3 mm.
Tensile properties are in general governed by the properties of the reinforcing fibres while compressive properties are in general governed by the properties of the resin matrix as well as the fiber/matrix interface between the reinforcing and the resin.
Microbuckling can occur when a composite material laminate is loaded in compression. Microbuckling is a phenomenon that occurs when the reinforcing fibres in the composite material buckle under the compressive load.
Microbuckling can start in a specific location for several reasons including local misalignment of the reinforcing fibres, lack of support of the reinforcing fibres, stress concentration etc. Once microbuckling is initiated, it can spread throughout the composite material leading to the formation of a band of damaged composite material also known as kink band. An example of microbuckling is shown in FIG. 1 of the accompanying drawings.
Another design allowable that is extensively used when designing with composite materials is Compression Strength After Impact (CAI or CSAI). CAI measures the ability of a composite material to tolerate damage. In the test to measure CAI, the composite material is subject to an impact of a given energy and then loaded in compression. Damage area and dent depth are measured following the impact and prior to the compression test. During this test, the composite material is constrained during the test to ensure that no elastic instability is taking place and the strength of the composite material is recorded.
There are several approaches known in the art that may be used. One approach to improve CAI by improving toughness, although development continues to find means of improving toughness, through particle toughening, thermoplastic, rubber, core-shell rubber, etc. particle toughening, interlaminar toughening using veils or films of thermoplastics, or other known toughening materials.
One such approach involves the use of insoluble or partially soluble particles to improve the CAI of composite materials. However, the prior art does not demonstrate the use of insoluble or partially soluble particles with specific properties to improve OHC. Specifically, using insoluble or partially soluble particles to improve CAI uses a thermoplastic polymer that is soluble in the resin, which phase separates upon curing.
Another known approach to improve the CAI of a composite material is to use particles to modify the interlaminar region between the different prepreg material plies. The particles improve the interlaminar toughness of the composite material leading to a reduction in the delamination area induced by the impact. The reduction in the delamination area translates in higher residual compressive strength.
Another important property of a composite material is the damage resistance. A composite material with high damage resistance will incur less damage from a given impact event. Instead, composite materials with high damage tolerance may incur different levels of physical damage but will retain high amounts of residual strength.
Limited research has been carried out on approaches to improve the notched compressive properties of composites materials. U.S. Pat. No. 5,985,431 is directed to the use of specific epoxy resin ingredients in a specific range in order to improve the Open Hole Compression performance. However, no examples on how to tailor a composite material in order to improve the notched properties are available in patent literature or publications.
The present invention surprisingly overcomes the problems of prior composite materials by using different prepreg materials within the same composite material to improve notched compressive properties, damage tolerance and damage resistance of the resulting composite material.